The use of polymer coatings to protect surfaces and enhance the aesthetic and functional properties of materials is well known. These polymer coatings are typically applied to surfaces as liquid systems using techniques such as rolling, brushing, sprinkling, casting and pneumatic or electrostatic spraying.
The rheological profile of the liquid coating systems on application is typically chosen such that the coating can be applied by the method of choice without problems, flow evenly over the substrate to which is applied, allowing surface unevenness introduced by the application step, as well as unevenness from the underlying substrate, to be leveled out as much as possible to create to best final appearance. At the same time, the liquid film should not be allowed to sag or form tears for non-horizontal substrates. Such sagging tends to be particularly pronounced for thicker films of coatings and also at any localized build-up of the film, such as at edges, holes and character lines in a substrate.
There is a need to control the fluidity of the applied coating film during its liquid stage—which includes any heating cycles following application that may be used to promote curing—such that enough leveling is obtained without detrimental sagging. This is particularly true for clear coatings that are used in automotive applications for example, where obtaining extremely well-leveled films without sagging complications is of the highest importance.
Rheology modification agents are normally introduced into coating compositions to create a pseudoplastic rheology profile, to limit or prevent sagging, and/or to avoid pigment settling for pigment containing formulations, or to optimize the orientation of pigments after application (such as metallic pigments in automotive basecoats), depending on the application for which the coating is to be used.
Urea based particulate materials form a known class of materials being able to create such a pseudoplastic rheology due to reversible flocculation. For example U.S. Pat. No. 4,311,622 discloses a sag control agent comprising a polyurea precipitated in the form of micro-disperse crystals. Also, U.S. Pat. No. 4,677,028 discloses a crystalline polyurea sag control agent that may be formed either in situ in the coating composition or externally and then added to the coating composition.
The suitability of a given rheology modification agent is dependent on many factors. It should be able to create an ideal rheological profile that itself is dependent on inter alia: the shear regime associated with the method by which the liquid coating is applied to a substrate; the thickness of the coating to be applied; the orientation of the substrate with respect to the horizontal; the surface tension; the colloidal interaction between particles solvents, resins and additives; the thermal regime associated with any concomitant curing of the coating composition after application; the amount and volatility of solvents applied; the response of viscosity to loss of solvents and raise in temperature; surface tension gradients developing upon solvent evaporation; the curing rate; and, the shear forces acting on the liquid coating. The agent must also be able to create enough robustness to allow for variations in these parameters while still maintaining results close to the optimal.
In addition to the rheological profile, there are further parameters that are of relevance for coating formulations containing rheology control agents, such as the lifetime of the coating formulation before application, and very importantly, the absence of any adverse optical effects introduced by these agents like color and haze, especially for applications such as clear coatings.
In addition to the rheological performance of a coating, its optical performance is also of the utmost importance. A sag control additive must be fine enough not to create any visible disturbance (such as protrusion) when applied in thin films. For clear coat applications, no detectable haze or turbidity should be present after completion of the curing cycle, and no color formation (or yellowing) should have resulted from its presence. Again, these characteristics should not change irreversibly with storage time.
Of course, in order to limit costs and the interference with other coating properties, and also to minimize the optical effects which are proportional to the amount of SCA added, efficient sag control agents that can do their job at low concentrations would be preferred.
Although many rheology control agents are known, none can fulfill all the demands listed above, and none so for all applications.
There has been a significant focus on the use as sag control agents of polyurea compounds derived from the reaction of an isocyanate component with an amine component. Based on the aforementioned reaction, the use of different amines and/or isocyanates will yield different polyurea compounds. If these compounds are allowed to precipitate, a range of forms, sizes and surface characteristics of the resultant products could be obtained, from isotropic to anisotropic forms and from fine micro-disperse crystals to fibrous forms, with a broad range of crystal stabilities (or “melting points”). Similarly, such distinctions in colloidal characteristics could be obtained where the batches of the same amines and isocyanates are reacted under different conditions, or in a different environment. Such characteristics are determinant of the rheological and optical properties of the polyurea compounds and their stabilities to given curing regimes.
At one extreme of morphology where the polyurea compounds crystallize in long acicular or fibrous forms, coating compositions comprising these fine structures have been shown to exhibit very low amounts of flow—and concomitantly sagging—when applied to surfaces as wet films at room temperature, allowing the use of low amounts of these materials to obtain the desired sagging limit reduction. Furthermore, where they are sufficiently fine, such polyurea compounds do not cause haze in the resultant coatings even when cured under very mild conditions, i.e. even if they stay present in the final coating without (reactive) dissolution in the curing cycle.
However, some of those urea compounds that produce such fine acicular or fibrous crystalline structures are based on expensive amine raw materials (e.g. chiral amines). Equally, such crystalline structures have been shown to exhibit in general relatively low dissolution temperatures in those solvents typically employed for coating compositions such that the rheological efficiency of the polyurea compounds during curing reactions at higher temperatures may be diminished due to a too high amount of flow allowed for in the latest stages of oven curing. Such a reduced dissolution point may also lower the resistance of the polyurea compound to aggressive solvents and cross-linking reagents in the coating compositions such that, although the polyurea compound may survive applications over short time-frames, its overall shelf-life or pot-life in such coating compositions may be limited.
Considering a different morphology, many polyurea compounds that crystallize in coarser structures—such as e.g. the most commonly used urea product based on benzylamine and HDI—do not tend to exhibit low dissolution temperatures and may thereby have an enhanced shelf-life. These coarse structures may however remain visible as haze if the cross-linking agent or curing conditions are too mild. Equally detrimentally, coarser crystalline structures are not as efficient rheologically at the temperatures and shear regimes of coatings applications. This is typically compensated for by utilizing increased amounts of the polyurea compounds in the sag control agent but this can further enhance the adverse optical effects. Moreover, the high stability of these structures may prohibit any flow occurring in the coating also in the late oven stages, thus also prohibiting leveling out of late unevenness caused by shrinkage of the film, telegraphing substrate unevenness.
It is clear that a given polyurea compound having specific crystalline and colloidal characteristics will have a defined package of both advantageous and disadvantageous rheological and optical properties. Consequently, there exists a need in the art to synergistically capture the advantageous properties of different crystalline structures of polyurea compounds while minimizing the disadvantages associated with said structures.